In a vehicle having a gasoline/electric hybrid engine, the vehicle may be powered alternately by either a gasoline-powered internal combustion engine or an electric motor to thereby optimize fuel efficiency while reducing vehicle emissions. Hybrid vehicles achieve a relatively high fuel efficiency or fuel economy by alternating between the gasoline-powered engine and the electric motor when one power source is better suited to a specific vehicle operating condition than the other. For example, a gasoline-powered engine is known to be more efficient than an electric motor during periods of constant or relatively non-variable vehicle speed, such as while cruising at a sustained rate of speed, while an electric motor is generally better suited than a gasoline engine for use when the vehicle power requirements are highly variable, such as during starting or stopping of the vehicle.
Voltage requirements on board a hybrid vehicle and some electric and fuel cell vehicles may range from a standard 12-volt (V) supply to more than 600V, depending on the electrical device being energized or driven. For example, a low-voltage (12V) electrical system typically powers standard vehicle accessories such as stereo systems, clocks, headlights, wiper blades, and/or horns, while high-voltage (>60V) direct-current (DC) circuits may be needed to charge a battery or a battery pack to a level sufficient for powering the vehicle's drive wheels. To provide and safely control such a wide range of on-board voltage options, hybrid vehicles are configured with appropriate circuitry, wiring, and other electrical components such as voltage inverters and converters, specially manufactured high-voltage batteries, controllers, and various sensors. The high-voltage vehicle circuits are isolated from both the vehicle chassis and the low-voltage circuits.